Jul 26, 2002

Impact of Mixing Chlorine and Chlorine Dioxide on Total Trihalomethane Formation - Part 1

The 1986 amendments to the Safe Drinking Water Act1 are
challenging many water utilities to meet stricter water quality requirements.
Two rules, the Surface Water Treatment Rule (SWTR)2 and the
Disinfection?Disinfection By-Products (D-DBP) Rule3, are requiring
utilities to implement more advanced technologies in water treatment. The
Surface Water Treatment Rule emphasizes the need for utilities to meet minimum
levels of disinfection for surface waters, whereas the Disinfection?Disinfection
By-Products Rule limits the disinfectant byproducts. Therefore, utilities will
have to implement a treatment approach that balances the benefits of
disinfection against disinfection byproducts.

After the Cryptosporidium outbreak in Milwaukee during March
1993, many utilities immediately began investigating the possible use of more
advanced disinfectants such as ozone and chlorine dioxide in order to combat
the threat from the Cryptosporidium protozoan. Chlorine alone was not adequate
to inactivate the new target organism. Most disinfectants produce disinfection
byproducts causing a potentially long-term adverse health impact from cancer.
Although chlorine has served the water industry well for about a century in
safeguarding the public?s health, it is relatively ineffective against
Cryptosporidium and can cause excessive trihalomethanes (THMs) in distribution
systems.5 THMs were limited in drinking water in 1979 to 100 ppb (THM Rule)
because of their potential carcinogenic properties.6

Although ozone is the strongest and most capable
disinfectant against Cryptosporidium, it can produce excessive bromates, a
potential carcinogen.7 Chlorine dioxide also is capable of inactivating
Cryptosporidium but not as well as ozone. However, there are concerns about its
disinfection by-products such as chlorites, identified as causing hemalytic
anemia, especially in 13 percent of black males.8 In El Paso, Texas, the
Umbenhauer/Robertson Water Plant is adding 2 mg/L of chlorine with 3 mg/L of
chlorine dioxide in the same disinfection zone in order to minimize TTHM
formation and maximize disinfection capability (Figure 1).

The primary purpose of this paper is to examine the impact
of adding chlorine with chlorine dioxide on the formation of TTHMs. Laboratory and
plant studies are presented to determine the impact of the addition of chlorine
alone and in combination with chlorine dioxide on the formation of TTHMs. Since
the Eka SVP-Pure Chlorine Dioxide Generator system is a chlorate-based
technology and does not have chlorine or chlorite in its generator stock
solution, it was used for testing.9

Laboratory Study Procedures

This project was carried out using laboratory studies.
Analyses were performed at the El Paso Water Utilities Central Laboratory and
an outside EPA-approved laboratory using proper QC/QA procedures. The raw water
source for the studies was the Rio Grande River. This water was available for
treatment during the period of March through September in El Paso. The raw
water samples were dosed with chlorine dioxide solutions obtained from the
chlorine dioxide generator utilizing a 40 percent sodium chlorate/50 percent
hydrogen peroxide/78 percent sulfuric acid system (Figure 2). In order to
evaluate the impact of adding chlorine with chlorine dioxide on disinfection
byproducts, raw water samples were dosed with 1, 2 or 3 mg/L of chlorine
dioxide and dosed with various amounts of chlorine ranging from 0 percent
(chlorine dioxide alone) to 200 percent of the chlorine dioxide dose. After the
sample sets were initially dosed with chlorine dioxide and chlorine, they were
held for about 45 minutes. Then, all of the sample sets were equally dosed with
7 mg/L of chlorine and held for a 1-hour contact period in order to compare the
effects of various combinations of chlorine dioxide and chlorine on TTHM

Comparison of the Effect of 1 mg/L of ClO2 and Chlorine
Doses on TTHMs

The results of these tests are shown in Figure 3. As
illustrated, the raw sample dosed with 7 mg/L chlorine alone, held for 1-hour
contact time, forms about 42 ppb of TTHMs. The 1 mg/L chlorine dioxide dose
with 0 percent chlorine had insignificant reduction in TTHMs. This is to be
expected because the initial chlorine dioxide demand (< 1 minute time) of
the Rio Grande River water is usually between 1.5 mg/L and 2.0 mg/L (i.e.,
there seems to be insufficient dosage to cause a long enough contact time of
the chlorine dioxide with TTHM precursors to cause a significant reduction in
TTHMs). However, when chlorine is added with the 1 mg/L of chlorine dioxide
dose, the TTHM levels are reduced at the 33 percent, 66 percent and 100 percent
chlorine levels. At the 150 percent and 200 percent levels, the TTHMs are
higher than the 0 percent chlorine and raw TTHM levels. The results imply that
the chlorine with 1 mg/L chlorine dioxide is participating in TTHM reduction by
possibly reforming chlorine dioxide with the chlorite by product of chlorine
dioxide degradation.

Since most water plants dose at the 1 mg/L chlorine dioxide
level, the perception from most water professionals is that chlorine dioxide at
this concentration can only prevent TTHM formation, not reduce TTHMs. However,
in this testing, it has been shown that TTHM reduction can be accomplished by
adding chlorine with chlorine dioxide, even at the 1 mg/L dosage level. When
the chlorite byproduct level is exceeded by the amount of the chlorine
necessary to reform chlorine dioxide, the excess chlorine will participate in
forming higher TTHMs. This also implies that chlorine is able to reduce the
chlorite byproduct levels in order to form chlorine dioxide. As shown in the
next section, the chlorite and chlorate levels were analyzed at various percent
chlorine dosages to determine if the chlorine was reducing chlorite.

Chlorine Effect on Chlorite and Chlorate at 1 mg/L

Chlorine Dioxide Dose

In Figure 4, the chlorite and chlorate levels versus
chlorine levels from 0 percent to 200 percent are plotted for the same samples
depicted in Figure 3. At 0 percent chlorine, the chlorite level is highest at
0.46 mg/L while the chlorate level is 0.19 mg/L. As the chlorine dosages are
increased, the chlorite levels decrease and the chlorate levels are rather
flat. The overall reduction in chlorite was about 0.10 mg/L (0.46 mg/L to 0.36
mg/L). The chlorate level increase was negligible at about 0.03 mg/L (0.19 mg/L
to 0.22 mg/L). Therefore, it seems reasonable that the chlorite reduction is
principally caused by the sequence of reactions with chlorine to reform
chlorine dioxide, ultimately being reduced to chloride. If the chlorine level
exceeds the amount needed by the chlorite level for reforming chlorine dioxide,
the amount of the chlorine not needed for chlorite oxidation will participate
in forming TTHMs. This may be the reason for the lowest TTHM level at about 66
percent chlorine and the subsequent increase in TTHMs at the 100 percent, 150
percent and 200 percent chlorine dosages.

Part 2 will appear in the September issue and will present
conclusions about the study.


Thanks are extended to the El Paso Water Utilities/Public
Service Board for funding and making their facilities available to complete
this study. Special thanks go to Richard Wilcox for his invaluable
contributions and outstanding laboratory work in analyzing our samples.

For a list of references, please go to

About the author

Douglas Rittmann, Ph.D., P.E., is a water/wastewater consultant and a lecturer at the University of Texas at El Paso Civil Engineering Department. He was previously the division manager for the El Paso Water Utilities.

Anthony Tarquin, Ph.D., P.E., is a professor in the Department of Civil Engineering at the University of Texas at El Paso.